The present invention relates generally to a Global Positioning System (GPS) receiver for satellite-based navigation systems such as the U.S. Global Positioning System, the Russian Global Navigation System or the like. More specifically, the present invention is directed to a GPS receiver configured to decode pseudo random noise (PRN) codes from broadcasted signals of the navigation systems.
Radio navigation systems such as the United States"" Global Positioning System (GPS), the Russian Global Navigation System (GLONASS) or the like are configured to broadcast signals that include navigation data encoded therein. The navigation data include each satellite""s ephemeris data, e.g., their positions and time indicated by their onboard navigational devices and clocks, respectively.
A conventional GPS receiver determines positions of the GPS satellites and transmission time of broadcasted signals therefrom by passively receiving and decoding the broadcasted signals. More specifically, the GPS receiver, by obtaining the arrival times of the broadcasted signals transmitted by the satellites in the receiver""s field of view, which are precisely measured relative to the receiver""s own clock, obtains its distances (ranges) to the respective satellites within a constant bias. The value of the constant bias is substantially equal to the difference between the satellites"" and the receiver""s clock times. Since the satellites"" clocks are synchronized closely to a system time, this constant bias is, within some very small error, substantially the same for all satellites. Distances between the GPS receiver and the satellites from which the received broadcasted signals sent are called pseudoranges, and they are offset from the actual distances by a constant value proportional to the constant bias. With four or more satellites in view, the four unknowns consisting of the receiver""s position (longitude, latitude, and elevation) and the receiver""s clock offset with respect to the system time can be solved using the measured receiver-to-satellite pseudoranges, and the satellites"" ephemeris data in the navigation data broadcasted by the satellites.
In order to allow the receiver to measure the pseudoranges, the satellites"" broadcasted signals are wide band pseudo random noise (PRN) coded signals. These PRN coded signals are radio frequency (RF) carriers modulated by a wide band PRN code, e.g., modulo-2 added to the navigation data. A unique PRN code is assigned to each satellite, and different types of PRN codes with different chip rates are used for different system applications. For instance, in the U.S. GPS, a 1 MHZ chip rate C/A code is used for initial acquisition and less accurate civilian applications, while a 10-MHZ chip rate P-code is used for higher accuracy military applications. The PRN codes are designed such that cross-correlation between different PRN codes from different satellites is minimized. For example, the GPS C/A codes are 1023 chip length Gold codes which have a maximum periodic cross-correlation value of only 65. This design feature allows the receiver to separate the ranging signals received from a number of GPS satellites by acquiring and tracking the unique PRN codes of the GPS satellites even though they are transmitted on the same RF frequency (1575.42 MHZ for GPS L1 and 1227.6 MHZ for GPS L2).
A conventional circuit used to track the PRN signals is a delay lock loop that correlates early, punctual, and late versions of locally generated PRN codes against the received signal. Further, the delay lock loop obtains an estimate of the time difference between the locally generated code and the received code from the difference between the correlation of the early version of the locally generated code against the received signal and the correlation of the late version of the locally generated code against the received signal.
The accuracy of the receiver is affected by several factors: the accuracy of the satellite ephemeris data and clock time given in the satellite""s navigation data, the propagation delays introduced by the ionosphere and the troposphere, receiver noise and quantization effects, radio frequency interferences, multipath effects, and the relative geometry of the satellites and the GPS receiver, which is measured in terms of geometric dilution of precision. Some of these error sources, in particular, the satellite ephemeris data and clock errors, the ionospheric and tropospheric delays, can be substantially eliminated in a differential GPS (DGPS) system, which can be either local area differential or wide area differential. (The Federal Aviation Agency (FAA) is developing both types of differential systems. Specifically, a Local Area Augmentation System (LAAS) is being developed for installation at airports for use in aircraft landing, and a Wide Area Augmentation System (WAAS) is being developed for en route navigation and lower accuracy landing requirements.)
In local area differential systems, the error in the determined position of a reference receiver, which is generally located at a precisely surveyed site, are subtracted from the position obtained by of the GPS receiver or, alternatively, the measured errors in the pseudoranges at the reference site are subtracted from the measured pseudoranges of the GPS receiver. For such local DGPS applications the GPS receiver is located usually within 10 to 50 kilometers of the reference receiver. Under these circumstances the error caused by the satellite ephemeris data, clock and by ionospheric and tropospheric delays are almost identical at both the user receiver and reference receiver, and are thus practically canceled in the correction process. The major error sources remaining are then receiver noise, receiver quantization and multipath effects. These effects are uncorrelated between the reference and user receivers, and are not canceled from each other in the correction process.
In wide area differential systems, the correlation of ionospheric, tropospheric and satellite orbital errors are decreased because of the increased distance between the reference receivers and the user receivers. To alleviate this decreased correlation, a wide area DGPS system, e.g., the FAA""s WAAS, will typically send separate corrections for the satellite clock errors, the satellite orbit errors and for the ionospheric refraction effects. The correction signals are derived from a network of ground reference stations that estimate the satellite clock and ephemeris data errors and the ionospheric refraction effects at specific grid points from which the user receiver can compute its specific expected errors. Similar to the local area DGPS systems, most of the error sources are mitigated in the correction process except for receiver noise, quantization noise, interference, and multipath effects. The effects of the receiver""s thermal and quantization noise can be reduced through averaging. The interference can be reduced with frequency management and regulation. The multipath effects thus become the most detrimental error source in DGPS systems. Sub-meter accuracy can generally be obtained by DGPS if the multipath error can be reduced sufficiently. However, multipath error in C/A code PRN tracking with early minus late discriminators can be as large as one chip (300 meters) and is usually several meters in magnitude. Reduction of multipath effects is thus an important design consideration in high quality GPS receivers.
The number of multipath signals, their relative delays and RF phase offsets with respect to the direct path signal are all functions of the satellite to the receiver""s antenna geometry relative to reflecting objects around the receiver antenna. Since multipath signals always travel a longer distance than the direct path signal, they are invariably delayed with respect to the direct path signal and will suffer a loss in power in the reflection process. If the multipath signal has a delay in excess of one PRN chip in time with respect to the direct path signal, it will not correlate with the locally generated code and will not affect the pseudorange measurement accuracy once the delay lock loop is locked onto the direct path signal. However, if the multipath delay with respect to the direct path signal is within one chip in time, the error signal measuring the relative time offset between the local code and the received signal in an early-minus-late discriminator configuration is usually biased by the multipath signal. For C/A code receivers this problem is significant since the C/A code chip time is one microsecond in length, allowing multipath signals to be delayed with respect to the direct path signal by as much 300 meters to influence the pseudorange measurement accuracy. In addition, since the chip time is equivalent to 300 meters in length, multipath error, even if a small fraction of a C/A chip, can be very detrimental. Multipath error is thus one of the larger error contributors in DGPS systems and considerable design effort has been expended to develop receivers which are resistant to the multipath effects.
There are a number of techniques which have been developed to minimize the errors due to multipath effects. These include: (1) careful site selection to minimize the number of signal reflectors in the nearby environment; (2) antenna design to decrease the sensitivity to signals arriving at the antenna from low elevation angles typical of reflected signals; and (3) by receiver processing techniques. Site selection is always recommended but natural ground reflections generally limit its usefulness. Antenna design can be of significant benefit but because of cost, size and weight problems such specific antenna design is generally limited to permanent reference receiver locations. As a result, multipath reduction by specific receiver design has received significant attention and a number of techniques have been developed. Specific techniques include: (a) narrowing the early minus late correlator spacing (Three documents describing this technique are: (1) U.S. Pat. No. 5,495,499, Feb. 27, 1996, Fenton et al; (2) Van Dierendonck et al. xe2x80x9cTheory and Performance of Narrow Correlator Spacing in a GPS Receiver,xe2x80x9d Navigation: Journal of the institute of Navigation, Vol. 39, No. 3, (Fall 1992); and (3) Hagerman, xe2x80x9cEffects of Multipath on Coherent and Noncoherent PRN Ranging Receivers,xe2x80x9d Aerospace Corporation Report No. TOR-0073(3020-03)-3 May 15, 1973); (b) specifically estimating the multipath error contribution by estimating the distortion of the correlation curve at multiple points and inferring from the distortion the magnitude and phasing of one or more reflected signals (see U.S. Pat. No. 5,414,729, May 9, 1995, Fenton); and (c) constructing special discriminator patterns sensitive to the rising edge of the correlator pattern which are less sensitive to multipath distortion because the discriminator is not affected by signals which arrive during the later portions of the correlation curve. (See U.S. Pat. No. 5,808,582, Feb. 5, 1997, Woo; and International Patent No. WO 96/37789, Nov. 28, 1996, Hatch et al.)
These different multipath reduction techniques all work with varying degrees of success and have been implemented in receivers by different manufacturers. However, these receivers with different correlator design have been found to present a potential safety problem to the FAA in its design of the high integrity WAAS and LAAS DGPS systems. GPS satellite PRN 19, not long after its launch, was observed to yield a significantly different pseudorange measurement for receivers with a 1.0 correlator spacing as compared to receivers with a 0.1 correlator spacing. Various fault modes in the satellite have been suggested. One of the more probable faults was an impedance mismatch at one of the signal cable connectors, causing reflections in the cable and a resultant broadcast signal with distortion similar to ground multipath effects. In any case, the PRN 19 satellite problem caused the FAA to initiate a study of potential distortions in the satellite signal which could distort the shape of the correlation curve such that receivers with different correlator spacing or different correlation discriminator design to yield different and possibly hazardous pseudorange measurements.
Two solutions have been suggested to solve the correlator sensitivity problem. The first solution is for the FAA to specify that the airborne receivers use the same correlator spacing and discriminator design as is used in the ground reference receivers. Existing WAAS ground reference receivers track the satellites with both a 1.0 correlator spacing and a 0.1 correlator spacing. Thus, this solution would involve the specification of either a 1.0 correlator spacing or a 0.1 correlator spacing in the airborne receiver. This solution is not favored by the airborne receiver designer for several reasons. First, the 1.0 correlator spacing is highly sensitive to multipath error and thus is clearly a substandard receiver. Second, the 0.1 correlator spacing has been patented by a Canadian company and is apparently not available for royalty free use. Third, the narrow correlator could still lead to hazardous pseudorange measurements if the satellite signal is distorted such that the narrow correlator could lock up on a sub-peak of the correlation curve. Fourth, the specification of specific receiver design does not allow innovation and product improvement. For example, the more effective multipath reduction techniques described above that observed the rising edges of the correlation curves would be specifically excluded.
The second solution available to the FAA is to monitor the shape of correlation curve at a sufficient number of points to ensure that the satellite is not broadcasting a signal with any significant distortion. This is, by far, a desirable solution. However, the phase one WAAS ground reference receivers are already in place, and, therefore, to modify those receivers to include correlators to monitor the correlation curve at more places is a very expensive proposition. But, for phase two WAAS implementation, replacing the existing ground reference receivers to monitor the entire correlation curve is highly desirable. In addition, the reference receivers for the LAAS systems installed for landing purposes at airports should also be capable of monitoring the entire correlation curve resulting from the broadcast satellite signals.
In view of the above, it will be appreciated that a receiver capable of monitoring the entire correlation curve, without the need to duplicate the receiver channel hardware for each correlator spacing or discriminator design, would be extremely beneficial and cost effective as a ground reference receiver and would eliminate the need to specify precisely the airborne receiver design.
The present invention provides a GPS receiver for reducing the multipath effects on coded signals and carrier phase measurements and for monitoring correlation curves of broadcasted signals from GPS satellites. The GPS receiver of the present invention processes the broadcast signal that includes a carrier frequency signal modulated by a Pseudo Random Code (PRN) signal. The receiver of the present invention includes an intermediate frequency (IF) processor configured to downconvert the broadcast signal to generate a first channel signal. The first channel signal is further downconverted by an angle rotator, to thereby recover the PRN signal from the broadcast signal. A carrier lock loop coupled to the angle rotator and configured to recover the carrier frequency signal is also provided. The recovered PRN signal is then processed by a signal generator configured to generate N gated PRN signals. The N gated PRN signals are generated based on a local replica PRN signal time-divided by M intervals within a chip period of the local replica PRN signal. The receiver of the present invention also includes a number of correlators each of which is configured to multiply a respective one of N gated PRN signals with a first phase signal of the PRN signal to generate a number of correlation values.
If desired the receiver of the present invention may also include a processor configured to adjust timing of the carrier lock loop based on the first plurality of correlation values in order to accurately track the carrier frequency signal.
The present invention is also directed to a method of processing the navigation broadcast signal that includes a carrier frequency signal modulated by a Pseudo Random Code (PRN) signal. The method includes the steps of downconverting the broadcast signal, to thereby recover the PRN signal from the broadcast signal, and generating N gated PRN signals, wherein the N gated PRN signals are generated based on a local replica PRN signal time-divided by M intervals within a chip period of the local replica PRN signal. Here, N and M are positive integers. The N gated PRN signals are then multiplied with a first phase signal of the PRN signal to generate a number of correlation values.
The method of the present invention also includes the step of adjusting timing of a phase lock loop based on the correlation values in order to accurately track the carrier frequency signal.